1. Field of the Invention
The present invention relates to an inverter circuit, or a switching device comprising upper and lower arms which is employed in an inverter of a converter circuit. Especially, the invention relates to a technique for driving the upper arm on the basis of a voltage across a capacitive element which is charged by the operation of the lower arm.
2. Description of the Background Art
There has been a previously known technique for driving a circuit to drive a switching element of the upper arm on the basis of the voltage across the capacitive element which is charged by the operation of a switching element of the lower arm.
FIG. 19 is a circuit diagram showing a switching technique disclosed for example in Japanese Patent Laid-open No. 3-150075A or 6-253553A. In FIG. 19, a capacitive element, namely capacitor 30, is charged with a voltage V.sub.D delivered by a power supply 15 via a blocking diode 22 when a switching element of the lower arm, namely insulated gate bipolar transistor (hereinafter referred to as "IGBT") 35, is conducting. When a switching element of the upper arm, namely IGBT 34, is conducting, the IGBT 35 of the lower arm is non-conducting. Thus, the voltage across the capacitor 30 is kept and on the basis of that voltage, an upper-arm driving circuit 25 drives the gate of the IGBT 34 of the upper arm.
The circuit of FIG. 19, however, has a first problem that the current charging the capacitor 30 becomes excessive in early stages of the operation because it is limited only by internal impedance of the power supply 15 and respective on-state impedance of the locking diode 22 and the IGBT 35 of the lower arm. This adversely affects the power supply 15.
A second problem is that part of a circulating current flowing from a load 40 to the capacitor 30 during the IGBT 34 of the upper arm is off becomes excessive. FIG. 20 is an equivalent circuit diagram for the circuit of FIG. 19 during both the IGBTs 34 and 35 are in the off state, i.e., in conditions of so-called interlock or dead band.
The circulating current I.sub.L from the load 40 branches to the free-wheeling diode 37 and to the capacitor 30. When the capacitor 30 has previously been charged by the conduction of the IGBT 35, a potential V.sub.C+ of the capacitor 30 closer to the blocking diode 22 and a potential V.sub.C- of the capacitor 30 far from the blocking diode 22 can be expressed as: EQU V.sub.C+ =V.sub.D -V.sub.22 ; EQU V.sub.C- =V.sub.35 &gt;0
where V.sub.22 is a threshold voltage of the blocking diode 22; and V.sub.35 is a saturation voltage of the IGBT 35 when turned on.
In this condition, when the circulating current I.sub.L flows from the load 40 to the free-wheeling diode 37, the free-wheeling diode 37 causes a voltage V.sub.F in proportion to the current flowing therethrough. Since the cathode of the free-wheeling diode 37 is directly connected to one end of the capacitor 30 far from the blocking diode 22, the potential V.sub.C- is immediately lowered from V.sub.35 (&gt;0) to -V.sub.F (&lt;0). This causes a large flow of charging current I.sub.ch from the power supply 15. Here the blocking diode 22 causes a voltage V.sub.f.
To resolve these problems, a technique for providing a current-limiting element, e.g., resistor, has also been proposed to prevent an excessive flow of current between the power supply 15 and the capacitor 30.
FIG. 21 is a circuit diagram showing a switching technique disclosed for example in Japanese Patent Laid-open No. 4-138068A. As compared with the circuit of FIG. 19, the circuit of FIG. 21 is configured in such a manner that a resistor 29 is inserted in series between the anode of the blocking diode 22 and the positive electrode of the power supply 15. The presence of the resistor 29 relieves the excessive flow of current, resolving the aforementioned first and second problems.
In the structure of FIG. 21, however, the third problem has arisen that the resistor 29 lowers a potential V.sub.B at the junction between the capacitor 30 and the cathode of the blocking diode 22, out of two potentials V.sub.B and V.sub.S on the basis of which the upper-arm driving circuit 25 drives the gate of the IGBT 34 of the upper arm.
FIG. 22 is an equivalent circuit diagram for the circuit of FIG. 21 during both the IGBTs 34 and 36 are off. When the blocking diode 22 causes the voltage V.sub.f in proportion to the charging current I.sub.ch from the power supply 15, the potential V.sub.B of the capacitor 30 is given by the following equation:
V.sub.B =V.sub.D -V.sub.f -I.sub.ch .times.R.sub.ch
where R.sub.ch is a resistance value of the resistor 29. As compared with the circuit of FIG. 20, the potential V.sub.B is lowered by I.sub.ch .times.R.sub.ch.
A noticeable decrease in the potential V.sub.B (third problem) may damage the upper-arm driving circuit 25.